Work uses an exchange 2010 for mail and calendaring. And with all the faults
exchange has with regard to handling Internet e-mail it is an ok calendar
solution. But I like Thunderbird as an e-mail client. Thunderbird has a
calendar plugin Lightning which gives Thunderbird full calendaring options.

The hard part is accessing exchange calendaring. But, there is a plugin which
does just that: Exchange 2007/2010 Calendar and Tasks Provider.
It takes a bit of fiddling to configure right (and the exchange server probably
has to allow this access via the web interface). And for some reason it
sometimes asks for my exchange password again and I have to cancel the first
request and answer the second.